In recent years, in applications of steel sheet for automobile use (for example, automobile pillars, door impact beams, bumper beams, etc.) and the like, steel sheet in which both high strength and high formability are achieved has been desired. As one means for dealing with this, there is TRIP (transformation induced plasticity) steel which utilizes the martensite transformation of residual austenite. Using this TRIP steel, it is possible to produce high strength steel sheet which is excellent in formability and which has a 1000 MPa class or so strength, but securing formability with very high strength steel sheet of further higher strength, for example, 1500 MPa or more, has been difficult.
In view of this situation, the forming method which has been focused on most recently as a method for securing high strength and high formability has been hot stamping (also called hot pressing, hot stamping, die quenching, press quenching, etc.) This hot stamping heats the steel sheet to the 800° C. or higher austenite region, then forms it by a die when hot to thereby improve the formability of the high strength steel sheet and, after forming it, cools it in the press die to quench it and thereby obtain a shaped part of the desired quality.
Hot stamping is promising as a method for forming very high strength members, but usually includes a step of heating the steel sheet in the atmosphere. At this time, oxides (scale) form on the steel sheet surface, so a later step of removing the scale becomes necessary. In this regard, in such a later step, there was the problem of the need for measures from the viewpoint of the descaling ability and environmental load etc.
As art to alleviate this problem, the art of using aluminum plated steel sheet as the steel sheet for hot stamped member use so as to suppress the formation of scale at the time of heating has been proposed (for example, see PLTs 1 and 2).
Aluminum plated steel sheet is effective for the efficient production of a high strength shaped part by hot stamping. Aluminum plated steel sheet is usually pressed formed, then painted. The aluminum plating layer after heating at the time of hot stamping changes to an intermetallic compound up to the surface. This compound is extremely brittle. If subjected to a severe forming operation by hot stamping, the aluminum plating layer easily cracks. Further, the phases of this intermetallic compound have more electropositive potential than the matrix steel sheet, so there was the problem that the corrosion of the steel sheet material is started from the cracks as starting points and the post painting anticorrosion property falls.
To avoid the drop in the post painting anticorrosion property due to the formation of cracks in the aluminum plating layer, adding Mn to this intermetallic compound is extremely effective, so an aluminum plated steel sheet which is improved in post painting anticorrosion property by addition of 0.1% or more of Mn in the aluminum plating layer has been proposed (for example, see PLT 3).
The art which is described in PLT 3 adds specific ingredient elements in the aluminum plating layer to prevent cracks from forming in the aluminum plating layer, but is not art which prevents cracks from forming in the aluminum plating layer without addition of specific ingredient elements into the aluminum plating layer.
Further, aluminum plated steel sheet has been proposed where, if adding elements to the matrix steel of the aluminum plated steel sheet to give Ti+0.1Mn+0.1Si+0.1Cr>0.25, these elements promote diffusion between Al—Fe so that even if cracks are formed in the aluminum plating layer, an Fe—Al reaction proceeds from around them and therefore the steel sheet material is prevented from being exposed and the corrosion resistance is improved (for example, see PLT 4).
However, the art which is described in PLT 4 does not try to prevent cracks from forming at the aluminum plating layer.